Single-Photon Detector-Based Interrogation For Distributed Fiber Optic Sensing Of Subsea Wells

ABSTRACT

A distributed acoustic system may comprise an interrogator which includes a single photon detector, an umbilical line comprising a first fiber optic cable and a second fiber optic cable attached at one end to the interrogator, and a downhole fiber attached to the umbilical line at the end opposite the interrogator. A method for optimizing a sampling frequency may comprise identifying a length of a fiber optic cable connected to an interrogator, identifying one or more regions on the fiber optic cable in which a backscatter is received, and optimizing a sampling frequency of a distributed acoustic system by identifying a minimum time interval that is between an emission of a light pulse such that at no point in time the backscatter arrives back at the interrogator that corresponds to more than one spatial location along a sensing portion of the fiber optic cable.

BACKGROUND

Boreholes drilled into subterranean formations may enable recovery ofdesirable fluids (e.g., hydrocarbons) using a number of differenttechniques. A number of systems and techniques may be employed insubterranean operations to determine borehole and/or formationproperties. For example, Distributed Acoustic Sensing (DAS) along with afiber optic system may be utilized together to determine borehole and/orformation properties. Distributed fiber optic sensing is acost-effective method of obtaining real-time, high-resolution, highlyaccurate temperature and strain (acoustic) data along at least a portionof the wellbore. In examples, discrete sensors, e.g., for sensingpressure and temperature, may be deployed in conjunction with the fiberoptic cable. Additionally, distributed fiber optic sensing may eliminatedownhole electronic complexity by shifting all electro-opticalcomplexity to the surface within the interrogator unit. Fiber opticcables may be permanently deployed in a wellbore via single- ordual-trip completion strings, behind casing, on tubing, or in pumpeddown installations; or temporally via coiled tubing, slickline, ordisposable cables.

Distributed sensing can be enabled by continuously sensing along thelength of the fiber, and effectively assigning discrete measurements toa position along the length of the fiber via optical time-domainreflectometry (OTDR). That is, knowing the velocity of light in fiber,and by measuring the time it takes the backscattered light to return tothe detector inside the interrogator, it is possible to assign adistance along the fiber.

Distributed acoustic sensing has been practiced for dry-tree wells, buthas not been attempted in wet-tree (or subsea) wells, to enableinterventionless, time-lapse reservoir monitoring via vertical seismicprofiling (VSP), well integrity, flow assurance, and sand control. Asubsea operation may utilize optical engineering solutions to compensatefor losses accumulated through long (˜5 to 100 km) lengths of subseatransmission fiber, 10 km of in-well subsurface fiber, and multiple wet-and dry-mate optical connectors, splices, and optical feedthroughsystems (OFS).

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred examples of the disclosure,reference will now be made to the accompanying drawings in which:

FIG. 1 illustrate an example of a well measurement system in a subseaenvironment;

FIG. 2 illustrates an example of a DAS system;

FIG. 3 illustrate the example of a DAS system with lead lines.

FIG. 4 illustrates a schematic of another example DAS system;

FIG. 5 illustrates an example of a remote circulator arrangement;

FIG. 6 illustrates a graph for determining time for a light pulse totravel in a fiber optic cable;

FIG. 7 illustrates another graph for determining time for a light pulseto travel in a fiber optic cable;

FIG. 8 illustrates an example of a remote circulator arrangement;

FIG. 9 illustrates another graph for determining time for a light pulseto travel in a fiber optic cable;

FIG. 10A illustrates a graph of sensing regions in the DAS system;

FIG. 10B illustrates a graph with an active proximal circulator using anoptimized DAS sampling frequency of 12.5 kHz;

FIG. 10C illustrates a graph with a passive proximal circulator using anoptimized DAS sampling frequency of 12.5 kHz;

FIG. 11 illustrates a graph of optimized sampling frequencies in the DASsystem;

FIG. 12 illustrates an example of a workflow for optimizing the samplingfrequencies of the DAS system;

FIG. 13 illustrates another example of the DAS system;

FIG. 14 illustrates another example of the DAS system;

FIG. 15 illustrates another example of the DAS system;

FIG. 16 illustrates an example of an interrogator in the DAS system;

FIG. 17 illustrates a schematic of the interrogator with a single photondetector;

FIG. 18 illustrates an examples of a single photon detector.

FIG. 19A-19D illustrates examples of a downhole fiber deployed in awellbore; and

FIG. 20 illustrates an example of the well measurement system in aland-based operation.

DETAILED DESCRIPTION

The present disclosure relates generally to a system and method forusing fiber optics in a DAS system in a subsea operation. Subseaoperations may present optical challenges which may relate to thequality of the overall signal in the DAS system with a longer fiberoptical cable. The overall signal may be critical since the end of thefiber contains the interval of interest, i.e., the well and reservoirsections. To prevent a drop in signal-to-noise (SNR) and signal quality,the DAS system described below may increase the returned signal strengthwith given pulse power, decrease the noise floor of the receiving opticsto detect weaker power pulses, maintain the pulse power as high aspossible as it propagates down the fiber, increase the number of lightpulses that can be launched into the fiber per second, and/or increasethe maximum pulse power that can be used for given fiber length.

FIG. 1 illustrates an example of a well system 100 that may employ theprinciples of the present disclosure. More particularly, well system 100may include a floating vessel 102 centered over a subterraneanhydrocarbon bearing formation 104 located below a sea floor 106. Asillustrated, floating vessel 102 is depicted as an offshore,semi-submersible oil and gas drilling platform, but could alternativelyinclude any other type of floating vessel such as, but not limited to, adrill ship, a pipe-laying slip, a tension-leg platforms (TLPs), a “spar”platform, a production platform, a floating production, storage, andoffloading (FPSO) vessel, and/or the like. Additionally, the methods andsystems described below nays also be utilized on land-based drillingoperations. A subsea conduit or riser 108 extends from a deck 110 offloating vessel 102 to a wellhead installation 112 that may include oneor more blowout preventers 114. In examples, riser 108 may also bereferred to as a flexible riser, flowline, umbilical and/or the like.Floating vessel 102 has a hoisting apparatus 116 and a derrick 118 forraising and lowering tubular lengths of drill pipe, such as a tubular120. I examples, tubular 120 may be a drill string, casing, productionpipe, and/or the like.

A wellbore 122 extends through the various earth strata toward thesubterranean hydrocarbon bearing formation 104 and tubular 120 may beextended within wellbore 122. Even though. FIG. 1 depicts a verticalwellbore 122, it should be understood by those skilled in the art thatthe methods and systems described are equally well suited for use inhorizontal or deviated wellbores. During drilling operations, the distalend of tubular 120, for example a drill sting, may include a bottom holeassembly (BHA) that includes a drill bit and a downhole drilling motor,also referred to as a positive displacement motor (“PDM”) or “mudmotor.” During production operations. tubular 120 may include a DASsystem. The DAS system may he inclusive of an interrogator 124,umbilical line 126, and downhole fiber 128.

Downhole fiber 128 may be permanently deployed in a wellbore via single-or dual-trip completion strings, behind casing, on tubing, or in pumpeddown installations. In examples, downhole fiber 128 may be temporarilydeployed via coiled tubing, wireline, slickline, or disposable cables.FIGS. 19A-19D illustrate examples of different types of deployment ofdownhole fiber 128 in wellbore 122 (e.g., referring to FIG. 1). Asillustrated in FIG. 19A, wellbore 122 deployed in formation 104 mayinclude surface casing 1900 in which production casing 1902 may bedeployed. Additionally, production tubing 1904 may be deployed withinproduction casing 1902. In this example, downhole fiber 128 may betemporarily deployed in a wireline system in which a bottom hole gauge1908 is connected to the distal end of downhole fiber 128. Furtherillustrated, downhole fiber 128 may be coupled to a fiber connection1906. Without limitation, fiber connection 1906 may attach downholefiber 128 to umbilical line 126 (e.g., referring to FIG. 1). Fiberconnection 1906 may operate with an optical feedthrough system (itselfcomprising a series of wet- and th-mate optical connectors) in thewellhead that optically couples downhole fiber 128 from the tubinghanger, to umbilical line 126 on the wellhead. instrument panel.Umbilical line 126 may include an optical flying lead, opticaldistribution system(s), umbilical termination unit(s), and transmissionfibers encapsulated in flying leads, flow lines, rigid risers, flexiblerisers, and/or one or more umbilical lines. This may allow for umbilicalline 126 to connect and disconnect from downhole fiber 128 whilepreserving optical continuity between the umbilical line 126 and thedownhole fiber 128.

FIG. 19B illustrates an example of permanent deployment of downholefiber 128. As illustrated in wellbore 122 deployed in formation 104 mayinclude surface casing 1900 in which production casing 1902. may bedeployed. Additionally, production tubing 1904 may be deployed withinproduction casing 1902. In examples, downhole fiber 128 is attached tothe outside of production tubing 1904 by one or more cross-couplingprotectors 1910. Without limitation, cross-coupling protectors 1910 maybe evenly spaced and may be disposed on every other joint of productiontubing 1904. Further illustrated, downhole fiber 128 may be coupled tofiber connection 1906 at one end and bottom hole gauge 1908 at theopposite end.

FIG. 19C illustrates an example of permanent deployment of downholefiber 128. As illustrated in wellbore 122 deployed in formation 104 mayinclude surface casing 1900 in which production casing 1902 may bedeployed. Additionally, production tubing 1904 may be deployed withinproduction casing 1902. In examples, downhole fiber 128 is attached tothe outside of production casing 1902 by one or more cross-couplingprotectors 1910. Without limitation, cross-coupling protectors 1910 maybe evenly spaced and may be disposed on every other joint of productiontubing 1904. Further illustrated, downhole fiber 128 may be coupled tofiber connection 1906 at one end and bottom hole gauge 1908 at theopposite end.

FIG. 19D illustrates an example of coiled tubing operation in whichdownhole fiber 128 may be deployed temporarily. As illustrated in FIG.19D, wellbore 122 deployed in formation 104 may include surface casing1900 in which production casing 1902 may be deployed. Additionally,coiled tubing 1912 may be deployed within production casing 1902. Inthis example, downhole fiber 128 may be temporarily deployed in a coiledtubing system in which a bottom hole gauge 1908 is connected to thedistal end of downhole fiber. Further illustrated, downhole fiber 128may be attached to coiled tubing 1912, which may move downhole fiber 128through production casing 1902. Further illustrated, downhole fiber 128may be coupled to fiber connection 1906 at one end and bottom hole gauge1908 at the opposite end. During operations, downhole fiber 128 may beused to take measurements within wellbore 122, which may be transmittedto the surface and/or interrogator 124 (e.g., referring to FIG. 1) inthe DAS system.

Additionally, within the DAS system, interrogator 124 may be connectedto an information handling system 130 through connection 132, which maybe wired and/or wireless. It should be noted that both informationhandling system 130 and interrogator 124 are disposed on floating vessel102. Both systems and methods of the present disclosure may beimplemented, at least in part, with information handling system 130.Information handling system 130 may include any instrumentality oraggregate of instrumentalities operable to compute, estimate, classify,process, transmit, receive, retrieve, originate, switch, store, display,manifest, detect, record, reproduce, handle, or utilize any form ofinformation, intelligence, or data for business, scientific, control, orother purposes. For example, an information handling system 130 may be aprocessing unit 134, a network storage device, or any other suitabledevice and may vary in size, shape, performance, functionality, andprice. Information handling system 130 may include random access memory(RAM), one or more processing resources such as a central processingunit (CPU) or hardware or software control logic, ROM, and/or othertypes of nonvolatile memory. Additional components of the informationhandling system 130 may include one or more disk drives, one or morenetwork ports for communication with external devices as well as aninput device 136 (e.g., keyboard, mouse, etc.) and video display 138.Information handling system 130 may also include one or more busesoperable to transmit communications between the various hardwarecomponents.

Alternatively, systems and methods of the present disclosure may beimplemented, at least in part, with non-transitory computer-readablemedia 140. Non-transitory computer-readable media 140 may include anyinstrumentality or aggregation of instrumentalities that may retain dataand/or instructions for a period of time. Non-transitorycomputer-readable media 140 may include, for example, storage media suchas a direct access storage device (e.g., a hard disk drive or floppydisk drive), a sequential access storage device (e.g., a tape diskdrive), compact disk, CD-ROM, DVD, RAM, ROM, electrically erasableprogrammable read-only memory (EEPROM), and/or flash memory; as well ascommunications media such as wires, optical fibers, microwaves, radiowaves, and other electromagnetic and/or optical carriers; and/or anycombination of the foregoing.

Production operations in a subsea environment present optical challengesfor DAS. For example, a maximum pulse power that may be used in DAS isapproximately inversely proportional to fiber length due to opticalnon-linearities in the fiber. Therefore, the quality of the overallsignal is poorer with a longer fiber than a shorter fiber. This mayimpact any operation that may utilize the DAS since the distal end ofthe fiber actually contains the interval of interest (i.e., thereservoir) in which downhole fiber 128 may be deployed. The interval ofinterest may include wellbore 122 and formation 104. For pulsed DASsystems such as the one exemplified in FIG. 2, an additional challengeis the drop-in signal to noise ratio (SNR) associated with the decreasein the number of light pulses that may be launched into the fiber persecond (pulse rate) when interrogating fibers with overall lengthsexceeding 10 km. As such, utilizing DAS in a subsea environment may haveto increase the returned signal strength with given pulse power,increase the maximum pulse power that may be used for given fiber opticcable length, maintain the pulse power as high as possible as itpropagates down the fiber optic cable length, and increase the number oflight pulses that may be launched into the fiber optic cable per second.

FIG. 20 illustrates an example of a land-based well system 2000, whichillustrates a coiled tubing operation. Without limitation, while acoiled tubing operation is shown, a wireline operation and/or the likemay be utilized. As illustrated interrogator 124 is attached toinformation handling system 130. Further discussed below, lead lines mayconnect umbilical line 126 to interrogator 124. Umbilical line 126 mayinclude a first fiber optic cable 304 and a second fiber optic cable 308which may be individual lead lines. Without limitation, first fiberoptic cable 304 and a second fiber optic cable 308 may attach to coiledtubing 2002 as umbilical line 126. Umbilical line 126 may traversethrough wellbore 122 attached to coiled tubing 2002. In examples, coiledtubing 2002 may be spooled within hoist 2004. Hoist 2004 may be used toraise and/or lower coiled tubing 2002 in wellbore 122. Furtherillustrated in FIG. 20, umbilical line 126 may connect to distalcirculator 312, further discussed below. Distal circulator 312 mayconnect umbilical line 126 to downhole fiber 128.

FIG. 2 illustrates an example of DAS system 200. DAS system 200 mayinclude information handling system 130 that is communicatively coupledto interrogator 124. Without limitation, DAS system 200 may include asingle-pulse coherent Rayleigh scattering system with a compensatinginterferometer. In examples, DAS system 200 may be used for phase-basedsensing of events in a wellbore using measurements of coherent Rayleighbackscatter or may interrogate a fiber optic line containing an array ofpartial reflectors, for example, fiber Bragg gratings.

As illustrated in FIG. 2, interrogator 124 may include a pulse generator214 coupled to a first coupler 210 using an optical fiber 212. Pulsegenerator 214 may be a laser, or a laser connected to at least oneamplitude modulator, or a laser connected to at least one switchingamplifier, i.e., semiconductor optical amplifier (SOA). First coupler210 may be a traditional fused type fiber optic splitter, a circulator,a PLC fiber optic splitter, or any other type of splitter known to thosewith ordinary skill in the art. Pulse generator 214 may be coupled tooptical gain elements (not shown) to amplify pulses generated therefrom.Example optical gain elements include, but are not limited to, ErbiumDoped Fiber Amplifiers (EDFAs) or Semiconductor Optical Amplifiers(SOAs).

DAS system 200 may include an interferometer 202. Without limitations,interferometer 202 may include a Mach-Zehnder interferometer. Forexample, a Michelson interferometer or any other type of interferometer202 may also be used without departing from the scope of the presentdisclosure. Interferometer 202 may include a top interferometer arm 224,a bottom interferometer arm 222, and a gauge 223 positioned on bottominterferometer arm 222. Interferometer 202 may be coupled to firstcoupler 210 through a second coupler 208 and an optical fiber 232.Interferometer 202 further may be coupled to a photodetector assembly220 of DAS system 200 through a third coupler 234 opposite secondcoupler 208. Second coupler 208 and third coupler 234 may be atraditional fused type fiber optic splitter, a PLC fiber optic splitter,or any other type of optical splitter known to those with ordinary skillin the art. Photodetector assembly 220 may include associated optics andsignal processing electronics (not shown). Photodetector assembly 220may be a semiconductor electronic device that uses the photoelectriceffect to convert light to electricity. Photodetector assembly 220 maybe an avalanche photodiode or a pin photodiode but is not intended to belimited to such.

When operating DAS system 200, pulse generator 214 may generate a firstoptical pulse 216 which is transmitted through optical fiber 212 tofirst coupler 210. First coupler 210 may direct first optical pulse 216through a fiber optical cable 204. It should be noted that fiber opticalcable 204 may be included in umbilical line 126 and/or downhole fiber128 (e.g., FIG. 1). As illustrated, fiber optical cable 204 may becoupled to first coupler 210. As first optical pulse 216 travels throughfiber optical cable 204, imperfections in fiber optical cable 204 maycause a portion of the light to be backscattered along fiber opticalcable 204 due to Rayleigh scattering. Scattered light according toRayleigh scattering is returned from every point along fiber opticalcable 204 along the length of fiber optical cable 204 and is shown asbackscattered light 228 in FIG. 2. This backscatter effect may bereferred to as Rayleigh backscatter. Density fluctuations in fiberoptical cable 204 may give rise to energy loss due to the scatteredlight, α_(scat), with the following coefficient:

$\begin{matrix}{\alpha_{scat} = {\frac{8\pi^{3}}{3\lambda^{4}}n^{8}p^{2}kT_{f}\beta}} & (1)\end{matrix}$

where n is the refraction index, p is the photoelastic coefficient offiber optical cable 204, k is the Boltzmann constant, and β is theisothermal compressibility. T_(f) is a fictive temperature, representingthe temperature at which the density fluctuations are “frozen” in thematerial. Fiber optical cable 204 may be terminated with a lowreflection device (not shown). In examples, the low reflection device(not shown) may be a fiber coiled and tightly bent to violate Snell'slaw of total internal reflection such that all the remaining energy issent out of fiber optical cable 204.

Backscattered light 228 may travel back through fiber optical cable 204,until it reaches second coupler 208. First coupler 210 may be coupled tosecond coupler 208 on one side by optical fiber 232 such thatbackscattered light 228 may pass from first coupler 210 to secondcoupler 208 through optical fiber 232. Second coupler 208 may splitbackscattered light 228 based on the number of interferometer arms sothat one portion of any backscattered light 228 passing throughinterferometer 202 travels through top interferometer arm 224 andanother portion travels through bottom interferometer arm 222.Therefore, second coupler 208 may split the backscattered light fromoptical fiber 232 into a first backscattered pulse and a secondbackscattered pulse. The first backscattered pulse may be sent into topinterferometer arm 224. The second backscattered pulse may be sent intobottom interferometer arm 222. These two portions may be re-combined atthird coupler 234, after they have exited interferometer 202, to form aninterferometric signal.

Interferometer 202 may facilitate the generation of the interferometricsignal through the relative phase shift variations between the lightpulses in top interferometer arm 224 and bottom interferometer arm 222.Specifically, gauge 223 may cause the length of bottom interferometerarm 222 to be longer than the length of top interferometer arm 224. Withdifferent lengths between the two arms of interferometer 202, theinterferometric signal may include backscattered light from twopositions along fiber optical cable 204 such that a phase shift ofbackscattered light between the two different points along fiber opticalcable 204 may be identified in the interferometric signal. The distancebetween those points L may be half the length of the gauge 223 in thecase of a Mach-Zehnder configuration, or equal to the gauge length in aMichelson interferometer configuration.

While DAS system 200 is running, the interferometric signal willtypically vary over time. The variations in the interferometric signalmay identify strains in fiber optical cable 204 that may be caused, forexample, by seismic energy. By using the time of flight for firstoptical pulse 216, the location of the strain along fiber optical cable204 and the time at which it occurred may be determined. If fiberoptical cable 204 is positioned within a wellbore, the locations of thestrains in fiber optical cable 204 may be correlated with depths in theformation in order to associate the seismic energy with locations in theformation and wellbore.

To facilitate the identification of strains in fiber optical cable 204,the interferometric signal may reach photodetector assembly 220, whereit may be converted to an electrical signal. The photodetector assemblymay provide an electric signal proportional to the square of the sum ofthe two electric fields from the two arms of the interferometer. Thissignal is proportional to:

$\begin{matrix}{{P(t)} = {{P1} + {P2} + {2*\sqrt{\left( {P\; 1P\; 2} \right){\cos \left( {{\varphi 1} - {\varphi 2}} \right)}}}}} & (2)\end{matrix}$

where P_(n) is the power incident to the photodetector from a particulararm (1 or 2) and ϕ_(n) is the phase of the light from the particular armof the interferometer. Photodetector assembly 220 may transmit theelectrical signal to information handling system 130, which may processthe electrical signal to identify strains within fiber optical cable 204and/or convey the data to a display and/or store it in computer-readablemedia. Photodetector assembly 220 and information handling system 130may be communicatively and/or mechanically coupled. Information handlingsystem 130 may also be communicatively or mechanically coupled to pulsegenerator 214.

Modifications, additions, or omissions may be made to FIG. 2 withoutdeparting from the scope of the present disclosure. For example, FIG. 2shows a particular configuration of components of DAS system 200.However, any suitable configurations of components may be used. Forexample, pulse generator 214 may generate a multitude of coherent lightpulses, optical pulse 216, operating at distinct frequencies that arelaunched into the sensing fiber either simultaneously or in a staggeredfashion. For example, the photo detector assembly is expanded to featurea dedicated photodetector assembly for each light pulse frequency. Inexamples, a compensating interferometer may be placed in the launch path(i.e., prior to traveling down fiber optical cable 204) of theinterrogating pulse to generate a pair of pulses that travel down fiberoptical cable 204. In examples, interferometer 202 may not be necessaryto interfere the backscattered light from pulses prior to being sent tophoto detector assembly. In one branch of the compensationinterferometer in the launch path of the interrogating pulse, an extralength of fiber not present in the other branch (a gauge length similarto gauge 223 of FIG. 1) may be used to delay one of the pulses. Toaccommodate phase detection of backscattered light using DAS system 200,one of the two branches may include an optical frequency shifter (forexample, an acousto-optic modulator) to shift the optical frequency ofone of the pulses, while the other may include a gauge. This may allowusing a single photodetector receiving the backscatter light todetermine the relative phase of the backscatter light between twolocations by examining the heterodyne beat signal received from themixing of the light from different optical frequencies of the twointerrogation pulses.

In examples, DAS system 200 may generate interferometric signals foranalysis by the information handling system 130 without the use of aphysical interferometer. For instance, DAS system 200 may directbackscattered light to photodetector assembly 220 without first passingit through any interferometer, such as interferometer 202 of FIG. 2.Alternatively, the backscattered light from the interrogation pulse maybe mixed with the light from the laser originally providing theinterrogation pulse. Thus, the light from the laser, the interrogationpulse, and the backscattered signal may all be collected byphotodetector assembly 220 and then analyzed by information handlingsystem 130. The light from each of these sources may be at the sameoptical frequency in a homodyne phase demodulation system, or may bedifferent optical frequencies in a heterodyne phase demodulator. Thismethod of mixing the backscattered light with a local oscillator allowsmeasuring the phase of the backscattered light along the fiber relativeto a reference light source.

FIG. 3 illustrates an example of DAS system 200 system, which may beutilized to overcome challenges presented by a subsea environment. DASsystem 200 may include interrogator 124 umbilical line 126, and downholefiber 128. As illustrated, interrogator 124 may include pulse generator214 and photodetector assembly 220, both of which may be communicativelycoupled to information handling system 130. Additionally,interferometers 202 may be placed within interrogator 124 and operateand/or function as described above. FIG. 3 illustrates an example of DASsystem 200 in which lead lines 300 may be used. As illustrated, anoptical fiber 212 may attach pulse generator 214 to an output 302, whichmay be a fiber optic connector. Umbilical line 126 may attach to output302 with a first fiber optic cable 304. First fiber optic cable 304 maytraverse the length of umbilical line 126 to a remote circulator 306.Remote circulator 306 may connect first fiber optic cable 3(4 to secondfiber optic cable 308. In examples, remote circulator 306 functions tosteer light unidirectionally between one or more input and outputs ofremote circulator 306. Without limitation, remote circulators 306 arethree-port devices wherein light from a first port is split internallyinto two independent polarization states and wherein these twopolarization states are made to propagate two different paths insideremote circulator 306. These two independent paths allow one or bothindependent light beams to be rotated in polarization state via theFaraday effect in optical media. Polarization rotation of the lightpropagating through free space optical elements within the circulatorthus allows the total optical power of the two independent beams touniquely emerge together with the same phase relationship from a secondport of remote circulator 306.

Conversely, if any light enters the second port of remote circulator 306in the reverse direction, the internal free space optical elementswithin remote circulator 306 may operate identically on the reversedirection light to split it into two polarizations states. Afterappropriate rotation of polarization states, these reverse in directionpolarized light beams, are recombined, as in the forward propagationcase, and emerge uniquely from a third port of remote circulator 306with the same phase relationship and optical power as they had beforeentering remote circulator 306. Additionally, as discussed below, remotecirculator 306 may act as a gateway, which may only allow chosenwavelengths of light to pass through remote circulator 306 and pass todownhole fiber 128. Second fiber optic cable 308 may attach umbilicalline 126 to input 309. Input 309 may be a fiber optic connector whichmay allow backscatter light to pass into interrogator 124 tointerferometer 202. Interferometer 202 may operate and function asdescribed above and further pass back scatter light to photodetectorassembly 220.

FIG. 4 illustrates another example of DAS system 200. As illustrated,interrogator 124 may include one or more DAS interrogator units 400,each emitting coherent light pulses at a distinct optical wavelength,and a Raman Pump 402 connected to a wavelength division multiplexer 40(WDM) with fiber stretcher. Without limitation, WDM 404 may include amultiplexer assembly that multiplexes the light received from the one ormore DAS interrogator units 400 and a Raman Pump 402 onto a singleoptical fiber and a demultiplexer assembly that separates themulti-wavelength backscattered light into its individual frequencycomponents and redirects each single-wavelength backscattered lightstream back to the corresponding DAS interrogator unit 400. In anexample, WDM 404 may utilize an optical add-drop multiplexer to enablemultiplexing the light received from the one or more DAS interrogatorunits 400 and a Raman Pump 402 and demultiplexing the multi-wavelengthbackscattered light received from a single fiber. WDM 404 may alsoinclude circuitry to optically amplify the multi-frequency light priorto launching it into the signal optical fiber and/or optical circuitryto optically amplify, the multi-frequency backscattered light returningfrom the single optical fiber, thereby compensating for optical lossesintroduced during optical (de-) multiplexing. Raman Pump 402 may be aco-propagating optical pump based on stimulated Raman scattering, tofeed energy from a pump signal to a main pulse from one or more DASinterrogator units 400 as the main pulse propagates down one or morefiber optic cables. This may conservatively yield a 3 dB improvement inSNR. As illustrated, Raman Pump 402 is located in interrogator 124 forco-propagation. in another example, Raman Pump 402 may be locatedtopside after one or more remote circulators 306 either in line withfirst fiber optic cable 304 (co-propagation mode) and/or in line withsecond fiber optic cable 308 (counter-propagation). In another example,Raman Pump 402 is marinized and located after distal circulator 312configured either for co-propagation or counter-propagation. In stillanother example, the light emitted by the Raman Pump 402. is remotelyreflected by using a wavelength-selective filter beyond a remotecirculator in order to provide amplification in the return path using aRaman Pump 402 in any of the topside configurations outlined above.

Further illustrated in FIG. 4. WDM 404 with fiber stretcher may attachproximal circulator 310 to umbilical line 126. Umbilical line 126 mayinclude one or more remote circulators 306, a first fiber optic cable304, and a second fiber optic cable 308. As illustrated, a first fiberoptic cable 304 and as second fiber optic cable 308 may be separate andindividual fiber optic cables that may be attached at each end to one ormore remote circulators 306. In examples, first fiber optic cable 304and second fiber optic cable 308 may be different lengths or the samelength and each may be an ultra-low loss transmission fiber that mayhave a higher power handling capability before non-literarily. This mayenable a higher gain, co-propagation Raman amplification frominterrogator 124.

Deploying first fiber optic cable 304 and as second fiber optic cable308 from floating vessel 102 (e.g., referring to FIG. 1) to a subseaenvironment to a distal-end passive optical circulator arrangement,enables downhole fiber 128, which is a sensing fiber, to be below aremote circulator 306 (e.g., well-only) that may be at the distal end ofDAS system 200. Higher (2-3×) pulse repetition rates, and non-saturated(non-back reflected) optical receivers may also be adjusted such thattheir dynamic range is optimized for downhole fiber 128. This mayapproximately yield a 3.5 dB improvement in SNR. Additionally, downholefiber 128 may be a sensing fiber that has higher Rayleigh scatteringcoefficient (i.e., higher doping) which may be result in a ten timesimprovement in backscatter, which may yield a 7-dB improvement in SNR.In examples, remote circulators 306 may further be categorized as aproximal circulator 310 and a distal circulator 312. Proximal circulator310 is located closer to interrogator 124 and may be located on floatingvessel 102 or within umbilical line 126. Distal circulator 312 may befurther away from interrogator 124 than proximal circulator 310 and maybe located in umbilical line 126 or within wellbore 122 (e.g., referringto FIG. 1). As discussed above, a configuration illustrated in FIG. 3may not utilize a proximal circulator 310 with lead lines 300.

FIG. 5 illustrates another example of distal circulator 312, which mayinclude two remote circulators 306. As illustrated, each remotecirculator 306 may function and operate to avoid overlap, atinterrogator 124, of backscattered light from two different pulses. Forexample, during operations, light at a first wavelength may travel frominterrogator 124 down first fiber optic cable 304 to a remote circulator306. As the light passes through remote circulator 306 the light mayencounter a Fiber Bragg Grating 500. In examples, Fiber Bragg Grating500 may be referred to as a filter mirror that may be a wavelengthspecific high reflectivity fitter mirror or filter reflector that mayoperate and function to recirculate unused light back through theoptical circuit for “double-pass” co/counter propagation induced DASsignal gain at 1550 nm. In examples, this wavelength specific “Ramanlight” mirror may be a dichroic thin film interference filter, FiberBragg Grating 500, or any other suitable optical filter that passes onlythe 1550 nm forward propagating DAS interrogation pulse light whilesimultaneously reflecting most of the residual Raman Pump light.

Without limitation, Fiber Bragg Grating 500 may be set-up, fabricated,altered, and/or the like to allow only certain selected wavelengths oflight to pass. All other wavelengths may be reflected back to the secondremote circulator, which may send the reflected wavelengths of lightalong second fiber optic cable 308 back to interrogator 124. This mayallow Fiber Bragg Grating 500 to split DAS system 200 (e.g., referringto FIG. 4) into two regions. A first region may be identified as thedevices and components before Fiber Bragg Grating 500 and the secondregion may be identified as downhole fiber 128 and any other devicesafter Fiber Bragg Grating 500.

Splitting DAS system 200 (e.g., referring to FIG. 4) into two separateregions may allow interrogator 124 (e.g., referring to FIG. 1) to pumpspecifically for an identified region. For example, the disclosed systemof FIG. 4 may include one or more pumps, as described above, placed ininterrogator 124 or after proximal circulator 310 at the topside eitherin line with first fiber optic cable 304 or second fiber optic cable 308that may emit a wavelength of light that may travel only to a firstregion and be reflected by Fiber Bragg Grating 500. A second pump mayemit a wavelength of light that may travel to the second region bypassing through Fiber Bragg Grating 500. Additionally, both the firstpump and second pump may transmit at the same time. Without limitation,there may be any number of pumps and any number of Fiber Bragg Gratings500 which may be used to control what wavelength of light travelsthrough downhole fiber 128. FIG. 5 also illustrates Fiber Bragg Gratings500 operating in conjunction with any remote circulator 306, whether itis a distal circulator 312 or a proximal circulator 310. Additionally,as discussed below, Fiber Bragg Gratings 500 may be attached at thedistal end of downhole fiber 218. Other alterations to DAS system 200(e.g., referring to FIG. 4) may be undertaken to improve the overallperformance of DAS system 200. For example, the lengths of first fiberoptic cable 304 and second fiber optic cable 308 selected to increasepulse repetition rate (expressed in terms of the time interval betweenpulses t_(rep)).

FIG. 6 illustrates an example of fiber optic cable 600 in which noremote circulator 306 may be used. As illustrated, at least a portion offiber optic cable 600 is a sensor and the pulse interval may be greaterthan the time for the pulse of light to travel to the end of fiber opticcable 600 and its backscatter to travel back to interrogator 124 (e.g.,referring to FIG. 1). This is so, since in DAS systems 200 at no pointin time, backscatter from more than one location along sensing fiber(i.e., downhole fiber 128) may be received. Therefore, the pulseinterval t_(rep) may be greater than twice the time light takes totravel “one-way” down the fiber. Let ts be the “two-way” time for lightto travel to the end of fiber optic cable 600 and back, which may bewritten as t_(rep)>t_(s).

FIG. 7 illustrates an example of fiber optic cable 600 with a remotecirculator 306 using the configuration shown in FIG. 3. When a remotecirculator 306 is used, only the light traveling in fiber optic cable600 that is allowed to go beyond remote circulator 306 and to downholefiber 128 may be returned to interrogator 124 (e.g., referring to FIG.1), thus, the interval between pulses is dictated only by the length ofthe sensing portion, downhole fiber 128, of fiber optic cable 600. Itshould be noted that all light must travel “to” and “from” the sensingportion, downhole fiber 128, with respect to pulse timing, what mattersis the total length of fiber “to” and “from” remote circulator 306.Therefore, first fiber optic cable 304 or second fiber optic cable 308may be longer than the other, as discussed above.

FIG. 8 illustrates an example remote circulator arrangement 800 whichmay allow, as described above, configurations that use more than oneremote circulator 306 close together at the remote location. Althoughremote circulator arrangement 800 may have any number of remotecirculators 306, remote circulator arrangement 800 may be illustrated asa single remote circulator 306.

FIG. 9 illustrates an example first fiber optic cable 304 and secondfiber optic cable 308 attached to a remote circulator 306 at each end.As discussed above, each remote circulator may be categorized as aproximal circulator 310 and a distal circulator 312. When using aproximal circulator 310 and a distal circulator 312, light from thefiber section before proximal circulator 310, and light from the fibersection below the remote circulator 306 are detected, which isillustrated in FIGS. 10 and 11. There is a gap 1000 between them of “nolight” that depends on the total length of fiber (summed) betweenproximal circulator 310 and a distal circulator 312.

Referring back to FIG. 9, with t_(s1) the duration of the light fromfiber sensing section before proximal circulator 310, t_(sep) the “deadtime” separating the two sections (and due to the cumulative length offirst fiber optic cable 304 and second fiber optic cable 308 betweenproximal circulator 310 and a distal circulator 312), and t_(s2) theduration of the light from the sensing fiber, downhole fiber 128, beyonddistal circulator 312, the constraints on fiber lengths and pulseintervals may be identified as:

$\begin{matrix}{{i.\mspace{11mu} t_{rep}} < t_{sep}} & (3) \\{{{ii}.\mspace{11mu} \left( {2t_{rep}} \right)} > \left( {t_{s1} + t_{sep} + t_{s2}} \right)} & (4)\end{matrix}$

Criterion (i) ensures that “pulse n” light from downhole fiber 128 doesnot appear while “pulse n+1” light from fiber before proximal circulator310 is being received at interrogator 124 (e.g., referring to FIG. 1).Criterion (ii) ensures that “pulse n” light from downhole fiber 128 isfully received before “pulse n+2” light from fiber before proximalcirculator 310 is being received at interrogator 124 is received. Itshould be noted that the two criteria given above only define theminimum and maximum t_(rep) for scenarios where two pulses are launchedin the fiber before backscattered light below the remote circulator 306is received. However, it should be appreciated that for those skilled inthe art these criteria maybe generalized to cases where n ∈{1,2,3, . ..} light pulses may be launched in the fiber before backscattered lightbelow the remote circulator 306 is received.

The use of remote circulators 306 may allow for DAS system 200 (e.g.,referring to FIG. 3) to increase the sampling frequency. FIG. 12illustrates workflow 1200 for optimizing sampling frequency when using aremote circulator 306 in DAS system 200. Workflow 1200 may begin withblock 1202, which determines the overall fiber length in bothdirections. For example, a 17 km of first fiber optic cable 304 and 17km of second fiber optic cable 308 before distal circulator 312 and 8 kmof sensing fiber, downhole fiber 128, after distal circulator 312, theoverall fiber optic cable length in both directions would be 50 km.Assuming a travel time of the light of 5 ns/m, the following equationmay be used to calculate a first DAS sampling frequency f_(s).

$\begin{matrix}{f_{s} = {\frac{1}{t_{s}} = \frac{1}{5 \cdot 10^{- 9} \cdot z}}} & (5)\end{matrix}$

where t_(s) is the DAS sampling interval and z is the overall two-wayfiber length. Thus, for an overall two way fiber length of 50 km thefirst DAS sampling rate f_(s) is 4 kHz. In block 1204 regions of thefiber optic cable are identified for which backscatter is received. Forexample, this is done by calculating the average optical backscatteredenergy for each sampling location followed by a simple thresholdingscheme. The result of this step is shown in FIG. 10A where boundaries1002 identify two sensing regions 1004. As illustrated in FIG. 10,optical energy is given as:

$\begin{matrix}{I^{2} + Q^{2}} & (6)\end{matrix}$

where I and Q correspond to the in-phase (I) and quadrature (Q)components of the backscattered light. In block 1206, the samplingfrequency of DAS system 200 is optimized. To optimize the samplingfrequency a minimum time interval is found that is between the emissionof light pulses such that at no point in time backscattered lightarrives back at interrogator 124 (e.g., referring to FIG. 1) thatcorresponds to more than one spatial location along a sensing portion ofthe fiber-optic line. Mathematically, this may be defined as follows.Let S be the set of all spatial sample locations x along the fiber forwhich backscattered light is received. The desired light pulse emissioninterval t_(s) is the smallest one for which the cardinality of the twosets S and {mod(x, t_(s)): x ∈ S} is still identical, which is expressedas:

$\begin{matrix}{{\min\limits_{t_{s}}{\left( t_{s} \right)\mspace{14mu} {s.t.\mspace{14mu} {S}}}} = {\left\{ {{{{mod}\left( {x,t_{s}} \right)}\text{:}\mspace{11mu} x} \in S} \right\} }} & (7)\end{matrix}$

where |·| is the cardinality operator, measuring the number of elementsin a set. FIG. 11 shows the result of optimizing the sampling frequencyfrom FIG. 10 with workflow 1200. Here, the DAS sampling frequency mayincrease from 4 kHz to 12.5 kHz without causing any overlap inbackscattered locations, effectively increasing the signal to noiseratio of the underlying acoustic data by more than 5 dB due to theincrease in sampling frequency.

Variants of DAS system 200 may also benefit from workflow 1200. Forexample, FIG. 13 illustrates DAS system 200 in which proximal circulator310 is placed within interrogator 124. This system set up of DAS system200 may allow for system flexibility on how to implement duringmeasurement operations and the efficient placement of Raman Pump 402. Asillustrated in FIGS. 13 and 14, first fiber optic cable 304 and secondfiber optic cable 308 may connect interrogator 124 to umbilical line126, which is described in greater detail above in FIG. 3.

FIG. 14 illustrates another example of DAS system 200 in which RamanPump 402 is operated in co-propagation mode and is attached to firstfiber optic cable 304 after proximal circulator 310. For example, if thefirst sensing region before proximal circulator 310 should not beaffected by Raman amplification. Moreover, Raman Pump 402, may also beattached to second fiber optic cable 308 which may allow the Raman Pump402 to be operated in counter-propagation mode. In examples, the RamanPump may also be attached to fiber 1400 between WDM 404 and proximalcirculator 310 in interrogator 124.

FIG. 15 illustrates another example of DAS system 200 in which anoptical amplifier assembly 1500 (i.e., an Erbium doped fiber amplifier(EDFA) +Fabry-Perot filter) may be attached to proximal circulator 310,which may also be identified as a proximal locally pumped opticalamplifier. In examples, a distal optical amplifier assembly 1502 mayalso be attached at distal circulator 312 on first fiber optical cable304 or second fiber optical cable 308 as an inline or “mid-span”amplifier. In examples, optical amplifier assembly 1502 located in-linewith fiber optical cable 304 and above distal circulator 312 may be usedto boost the light pulse before it is launched into the downhole fiber128. Referring to FIGS. 10B and 10C, the effect of using an opticalamplifier assembly 1500 in-line with a second fiber optic cable 308prior to proximal circulator 310 and/or using an distal opticalamplifier assembly 1502 located in line with second fiber optical cable308 above distal circulator 312 may allow for selectively amplifying thebackscattered light originating from downhole fiber 128 which tends tosuffer from much stronger attenuation as it travels back along downholefiber 128 and second fiber optical cable 308 than backscattered lightoriginating from shallower sections of fiber optic cable that may alsoperform sensing functions. FIG. 10B illustrates measurements whereproximal circulator 310 is active (optical amplifier assembly 1500in-line with a second fiber optic cable 308 prior to proximal circulator310 and/or distal optical amplifier assembly 1502 located in line withsecond fiber optical cable 308 above distal circulator 312 is used).FIG. 10C illustrates measurements where proximal circulator 310 ispassive (no optical amplification is used in-line with second fiberoptic cable 308). In FIGS. 10B and 10C, boundaries 1002 identify twosensing regions 1004. Additionally, in FIGS. 10B and 10C the DASsampling frequency is set to 12.5 kHz using workflow 1200. Furtherillustrated Fiber Bragg Grating 500 may also be disposed on first fiberoptical cable 304 between distal optical amplifier assembly 1502 anddistal circulator 312.

FIG. 16 illustrates an example schematic view of interrogator 124. Asillustrated interrogator 124 may be connected to umbilical line 126 anddownhole fiber 128 to form DAS system 200. As illustrated, umbilicalline 126 may include any number of distal circulators 312 and downholefiber 128 may include an optional Raman Mirror, which may also bereferred to as Fiber Bragg Grating 500.

Interrogator 124 may include one or more lasers 1600. Lasers 1600 may bemultiplexing laser, which may operate by multiplexing a pluralitycoherent laser sources via a WDM 404. One or more lasers 1600 may emit alight pulse 1602, which may be of a modified pulse shape. Optical pulseshaping and pre-distortion methods may be employed to increase overalloptical power that may be launched into a fiber string 1604, which mayconnect one or more lasers 1600 to proximal circulator 310. Light pulse1602 may travel from proximal circulator 310 through first fiber opticcable 304 to WDM 404, which may be attached to a Raman Pump 402 at theopposite end, and to umbilical line 126. Light pulse 1602 may travel todistal circulator 312 in umbilical line 126 and the length of downholefiber 128. Any residual Raman amplification may be reflected back byFiber Bragg Grating 500 that has been constructed to reflect theparticular wavelengths used by the Raman Pump and transmit all others.The backscattered light from the downhole fiber 128 may travel back todistal circulator 312 and then up second fiber optic cable 308 to adedicated interrogator receiver arm.

In examples, the dedicated interrogator receiver arm may allowinterrogator 124 to selectively receive backscattered light fromdifferent portions along the length of a fiber optic cable, as seen inFIGS. 10 and 11. For example, interrogator receiver arm may include adedicated amplifier 1608 that may selectively amplify the backscatteredlight from downhole fiber 128, a second region of the fiber optic cable,using a higher amplification factor than the dedicated amplifier 1608used to selective amplify the backscattered light received from firstfiber optic cable 304, a first region of the fiber optic cable. Gauges1612 may have gauge lengths employed in the two dedicated interrogatorreceiver arms may differ (e.g., also described in FIG. 2). Finally, eachdedicated interrogator receiver arm may be equipped with receivers 1606that are optimized according to certain characteristics of theinterferometric signals corresponding to the backscattered lightreceived from the two fiber sensing regions. Note that although FIG. 16only shows two dedicated interrogator receiver arms for each sensingfiber regions, it is not intended to be limited to such and may beextended to an arbitrary number of dedicated interrogator receiver arms,where each receiver arm receives and processes the backscattered lightsignal of a single sensing fiber region of downhole fiber 128.

FIG. 16 further illustrates example inputs 1610 for piezoelectric (PZT)devices. In examples, PZT devices functionally allow dynamic stretching(straining) of optical fibers, which may be embodied in coiled formaround the PZT, attached thereto, resulting in optical phase modulationof light propagating along the attached optical fiber. The PZT elementsare excitable via electrical signals from any electronic signalinformation generating source thus allowing information to be convertedfrom electrical signals to optical phase modulated signals along theoptical fiber attached thereto. Without limitation, PZT devices attachedto input 1610 may be a GPS receiver, seismic controller, hydrophone,and/or the like.

FIG. 17 illustrates an example of a schematic view of interrogator 124with a single photon detector (SPD) 1700. As discussed above, referringback to FIG. 16, devices such as Raman Pump 402 and amplifier 1608 areutilized in interrogator 124, or within DAS system 200 to amplify lightpulse 1602 to increase SNR. Additionally, modification to DAS system200, as discussed above, to further include Fiber Bragg Grating 500,different fiber lengths for first fiber 304 and second fiber optic cable308, and the use of remote circulators 306 to increase SNR.

Utilization of SPD 1700 alters DAS system 200 by reducing the noisefloor with DAS system 200 to increase SNR. The noise floor is theaverage energy over a spectral range generated by background processesin the detection system. For an optical device, these may includethermal noise (due to fluctuations caused by heat), pink noise (due tofluctuations caused by changing defects), burst noise (due tofluctuations caused by static defects), and shot noise (due to intrinsicfluctuations of the electromagnetic field with the detector). An SPD1700 may eliminate (through reduction or compensation) all sources ofnoise except shot noise and may lead to a reduction in the noise floorby up to 100 dB, directly increasing SNR.

In examples, SPD 1700 may be used in subsea operation or land operationsutilizing Rayleigh DAS, Raman Distributed Temperature Sensing (DTS), andBrillouin Distributed Strain Sensing (DSS). DTS operates and functionswhen a light pulse generates backscattered signals due to inelasticscattering within optical fiber. This inelastic scattering, which isstrongly temperature dependent, results in a frequency shift to lowerfrequency (Stokes Raman Scattering) or higher frequency (Anti-StokesRaman Scattering), both of which are temperature dependent (and usuallyaround ˜13 THz). By detecting these two shifted back-scattered signals,and appropriate math, the temperature may be determined. DSS operatesand functions on a photon inelastically interacting with an acousticphonon in an optical fiber. During the interaction, momentum istransferred with the phonon and the backscattered photon is frequencyshifted (˜9-11 GHz) compared to the incident light frequency. The extentof frequency shift is dependent on the strain and the temperature of thefiber.

SPD 1700 may be cyro-cooled and operate and function utilizingsuperconducting nanowire technology. In examples SPD 1700 does notrequire boosting of optical power but rather lowers the noise floor ofsignal detection by up to a factor of 100 dB. The detector in SPD 1700may be designed to multiplex multiple wavelengths or polarizations intothe same detector system and may have very narrow wavelength selectivityor larger optical linewidths. These allow both strong wavelengthselectivity without the need of optical filters or enables detection ofmultiple backscatter pulse types (Raman, Brillouin, Rayleigh) on thesame detector system. An SPD 1700 may include superconducting nanowiresingle-photon detector, Photomultiplier tubes, Avalanche photodiodes,Frequency up-conversion, Visible light photon counter, Transition edgesensor, Quantum dots, and Perovskite/Graphene phototransistors (for roomtemperature operation). In examples, Multiple SPDs and beam-splittersmay be used, such as in a homodyne configuration comparing the sum anddifferences of two SPD signals after a beam path is split by thebeamsplitter), to determine the extent of the contribution of shot noiseof the overall signal. In examples, the quantum efficiency of SPDs mayrange from ˜20% up to 99.99%

FIG. 18 illustrates an example of a schematic drawing of SPD 1700. Asillustrated, SPD 1700 may include a housing 1800 for enclosing theoptical detector 1802 and for providing an optical shield for opticaldetector 1802. Housing 1800 may include an aperture 1804 for passage ofthe fiber optic cable, which is identified as second fiber optic cable308. However, examples are not limited thereto, and in some examples, acoupler may be mounted so that second fiber optic cable 308 terminatesat a boundary of the housing 1800.

In examples, SPD 1700 may include a cooling mechanism 1806 having thehousing 1800 mounted thereto. Cooling mechanism 1806 is configured tomaintain the temperature of a light-sensitive region of optical detector1802 within a temperature range below 210 degrees Kelvin. In someexamples, cooling mechanism 1806 operates using liquid helium (He) orliquid nitrogen (N2). In some examples, cooling mechanism 1806 maintainsthe temperature of the light-sensitive region of optical detector 1802at a temperature at or below 80 degrees Kelvin. In some examples,cooling mechanism 1806 maintains the temperature of the light-sensitiveregion of the optical detector 1802 at a temperature at or below 5degrees Kelvin (e.g., when sealed helium systems are used). In someexamples, cooling mechanism 1806 may be of one or more of a variety ofconfigurations, including Dilutio-Magnetic, Collins-Helium Liquefier,Joule-Thomson, Stirling-cycle cryocooler, self-regulated Joule-Thomson,Closed-Cycle Split-Type Stirling, Pulse Tube, a two-stageGifford-McMahon cryogenic cooler or multi-stage Gifford-McMahoncryogenic cooler, or a cooler using magnetocaloric effect, by way ofexample. Lowering the temperature of optical detector 1802 improves theSNR of optical detector 1802 by decreasing dark current, by increasingsensitivity, and by reducing resistive loss by causing optical detector1802 to enter a superconducting regime of operation. In some embodimentsor configurations non-SPD optical detectors 1802 may not enter asuperconducting regime, while still having little to no thermal noise.

In some examples, SPD 1700 includes a cold head 1808 between the opticaldetector 1802 and cooling mechanism 1806. However, some embodiments donot include cold head 1808. In examples, housing 1800 is mounted tocooling mechanism 1806 such that moisture is prevented from entering thehousing. For example, housing 1800 may be mounted such that a vacuumseal is formed with the cooling mechanism 1806 or the cold head 1808.Additionally, housing 1800 may have a non-reflective inner surface.

As further illustrated in FIG. 18, SPD 1700 may further include aswitching or splitting mechanism 1810 to direct optical signals tooptical detector 1802, or a non-SPD optical detector 1812. Splittingmechanism 1810 may split optical signals based at least in part onwavelength of the optical signal, power of the optical signal,polarization, or any other parameter or criterion. For example,high-power optical signals may be routed to non-SPD optical detector1812, and away from optical detector 1802 and low-powered opticalsignals may be routed to optical detectors 1802. This routing may beperformed to prevent damage to optical detector 1802 while still takingfull advantage of LLD and ELLD capabilities of optical detector 1802.Without limitation, high-power optical signals may cause saturation inoptical detector 1802, leading to damage to optical detector 1802 or toinaccurate results. In some examples, saturation of optical detector1802 may occur with optical signal inputs having a power of about 100microwatts, and damage may occur at about 10 milliwatts. The noise floorthat may be detected by optical detector 1802 may be at a level slightlybelow saturation level but is typically at least 20-30 dB. Thesaturation level and noise floors for non-SPD optical detectors 1812 maybe different from the saturation level and noise floors for opticaldetector 1802. The saturation levels and noise floors also may or maynot overlap, and thus multiple types of detectors may be used that maycover the full power range for system measurements. For at least thesereasons, to measure a larger range of possible optical signals, opticaldetector 1802 are used in a system with non-SPD optical detectors 1812.Splitting mechanisms 1810 may direct or reroute optical signals based onpower level or other criteria, to take advantage of the different powerranges measurable by optical detector 1802 versus non-SPD opticaldetectors 1812.

In addition to or instead of a splitting mechanism 1810, SPD 1700 mayinclude a coupling mechanism or other mechanism to split the light withoptical couplers (with or without feedback). These mechanisms may bemulti-stage (e.g., the light may be split in one stage, then split againin a second stage), and may split light based on power, wavelength, orphase. Processor or computation-based systems may also be used in someembodiments to dynamically direct or reroute light signals among anyavailable optical path as power increases or based on any othercriteria.

In examples, SPD 1700 may be connected to information handling system130 (e.g., referring to FIG. 1) through interrogator 124 (e.g.,referring to FIG. 1) to obtain measurement data. In some examples, someportions of the interrogator 124 may be positioned at a surface of theEarth, while some portions to interrogator 124 may be placed downhole.When more than one optical detector 1802 is used, for example, some ofthe optical detectors 1802 or 1812 may be placed downhole, and some maybe placed at the surface. In some examples, one or more coolingmechanisms 1806 may be placed downhole proximate one or more opticaldetectors 1802 although power and geometry considerations should betaken into account with such configurations to provide power for coolingin an appropriately sized borehole.

In production and/or measurement operations, the use of SPD 1700 may besafer than using a Raman Pump 4024 (e.g., referring to FIG. 16). RamanPump 402 may increase the power moving through DAS system 200, which maylead to explosions and damage from high power increased by Raman Pump402. Using SPD 1700 removes Raman Pump 402 and protects againstexplosions from hazardous gas used with Raman Pump 402, increases eyesafety by prevent high energy light pulses from contacting the humaneye, and may further prevent connector damage and failure from highpower densities. Additionally, as lower optical pulse powers are used,non-linear distortion of the optical pulse shape is negligible, allowingfor minimal to no pulse forming.

Utilizing SPD 1700 may improve current technology by allowing greaterlengths of fiber with greatly attenuated signals, high transmission lossinterconnects (such as used offshore) may be used, even though theattenuation is high. An SPD 1700 may have selective frequency, reducingbackground noise contribution of other optical sources or devices (suchas from a Raman pump or scatter from a grating), and distortion of theoptical pulse shape is negligible. Additionally, an SPD 1700 may begated extremely fast, detect very few photons, and the spatialresolution can be extremely high.

The systems and methods for using a distributed acoustic system in asubsea environment may include any of the various features of thesystems and methods disclosed herein, including one or more of thefollowing statements.

The systems and methods for a DAS system within a subsea environment mayinclude any of the various features of the systems and methods disclosedherein, including one or more of the following statements.

Statement 1. A distributed acoustic system (DAS) may comprise aninterrogator which includes a single photon detector, an umbilical linecomprising a first fiber optic cable and a second fiber optic cableattached at one end to the interrogator, and a downhole fiber attachedto the umbilical line at the end opposite the interrogator.

Statement 2. The DAS of statement 1, wherein the single photon detectoris attached to a cooling mechanism.

Statement 3. The DAS of statements 1 or 2, wherein the first fiber opticcable is connected to a distal circulator.

Statement 4. The DAS of statement 3, further comprising a second fiberoptic cable connected to the distal circulator.

Statement 5. The DAS of statement 4, wherein the first fiber optic cableand the second fiber optic cable are different lengths.

Statement 6. The DAS of statements 1-3, further comprising a proximalcirculator and a distal circulator and wherein one or more remotecirculators form the proximal circulator or the distal circulator.

Statement 7. The DAS of statement 6, further comprising at least oneFiber Bragg Grating attached to the proximal circulator or the distalcirculator.

Statement 8. The DAS of statements 1-3 or 6, wherein the interrogator isconfigured to receive backscattered light from a first sensing regionand a second sensing region.

Statement 9. The DAS of statement 8, wherein an interrogator receiverarm is configured to receive the backscattered light from the firstsensing region or the second sensing region.

Statement 10. The DAS of statements 1-3, 6, or 8, wherein the DAS isdisposed in a subsea system operation of one or more wells and theumbilical line attaches to the downhole fiber at a fiber connection.

Statement 11. An interrogator may comprise one or more lasers configuredto emit one or more light pulses, an umbilical line connected to the oneor more lasers at one end and a downhole fiber at an opposite end andwherein the umbilical line comprises a first fiber optic cable and asecond fiber optic cable, and a single photon detector connected to thesecond fiber optic cable.

Statement 12. The interrogator of statement 11, further comprising aproximal circulator located between the one or more lasers and the firstfiber optic cable.

Statement 13. The interrogator of statements 11 or 12, furthercomprising a cooling mechanism connected to the single photon detector.

Statement 14. The interrogator of statements 11-13, further comprisingone or more piezoelectric inputs.

Statement 15. The interrogator of statements 11-14, wherein the firstfiber optic cable and the second fiber optic cable are attached to adistal circulator and the distal circulator is disposed in the umbilicalline.

Statement 16. A method for optimizing a sampling frequency may compriseidentifying a length of a fiber optic cable connected to aninterrogator, wherein the interrogator includes a single photon detectorconfigured to receive a backscatter by detection with the single photondetector, identifying one or more regions on the fiber optic cable inwhich a backscatter is received, and optimizing a sampling frequency ofa distributed acoustic system (DAS) by identifying a minimum timeinterval that is between an emission of a light pulse such that at nopoint in time the backscatter arrives back at the interrogator thatcorresponds to more than one spatial location along a sensing portion ofthe fiber optic cable.

Statement 17. The method of statement 16, wherein the fiber optic cablecomprises an umbilical line connected to a downhole fiber through afiber connection.

Statement 18. The method of statements 16 or 17, further comprisingdetermining an optical energy of the backscatter.

Statement 19. The method of statements 16-18, wherein the fiber opticcable comprises an umbilical line and the umbilical line comprises afirst fiber optic cable and a second fiber optic cable both attached toa distal circulator.

Statement 20. The method of statements 16-19, wherein the interrogatorcomprises one or more receivers.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations may be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. The precedingdescription provides various examples of the systems and methods of usedisclosed herein which may contain different method steps andalternative combinations of components. It should be understood that,although individual examples may be discussed herein, the presentdisclosure covers all combinations of the disclosed examples, including,without limitation, the different component combinations, method stepcombinations, and properties of the system. It should be understood thatthe compositions and methods are described in terms of “comprising,”“containing,” or “including” various components or steps, thecompositions and methods can also “consist essentially of” or “consistof” the various components and steps. Moreover, the indefinite articles“a” or “an,” as used in the claims, are defined herein to mean one ormore than one of the element that it introduces.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as, rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present examples are well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular examples disclosed above are illustrative only, and may bemodified and practiced in different but equivalent manners apparent tothose skilled in the art having the benefit of the teachings herein.Although individual examples are discussed, the disclosure covers allcombinations of all of the examples. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. Also, the terms in the claimshave their plain, ordinary meaning unless otherwise explicitly andclearly defined by the patentee. It is therefore evident that theparticular illustrative examples disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of those examples. If there is any conflict in the usages of aword or term in this specification and one or more patent(s) or otherdocuments that may be incorporated herein by reference, the definitionsthat are consistent with this specification should be adopted.

What is claimed is:
 1. A distributed acoustic system (DAS) comprising:an interrogator which includes a single photon detector; an umbilicalline comprising a first fiber optic cable and a second fiber optic cableattached at one end to the interrogator; and a downhole fiber attachedto the umbilical line at the end opposite the interrogator.
 2. The DASof claim 1, wherein the single photon detector is attached to a coolingmechanism.
 3. The DAS of claim 1, wherein the first fiber optic cable isconnected to a distal circulator.
 4. The DAS of claim 3, furthercomprising a second fiber connected to the distal circulator.
 5. The DASof claim 4, wherein the first fiber optic cable and the second fiberoptic cable are different lengths.
 6. The DAS of claim 1, furthercomprising a proximal circulator and a distal circulator and wherein oneor more remote circulators form the proximal circulator or the distalcirculator.
 7. The DAS of claim 6, further comprising at least one FiberBragg Grating attached to the proximal circulator or the distalcirculator.
 8. The DAS of claim 1, wherein the interrogator isconfigured to receive backscattered light from a first sensing regionand a second sensing region.
 9. The DAS of claim 8, wherein aninterrogator receiver arm is configured to receive the backscatteredlight from the first sensing region or the second sensing region. 10.The DAS of claim 1, wherein the DAS is disposed in a subsea systemoperation of one or more wells and the umbilical line attaches to thedownhole fiber at a fiber connection.
 11. An interrogator comprising:one or more lasers configured to emit one or more light pulses; anumbilical line connected to the one or more lasers at one end and adownhole fiber at an opposite end and wherein the umbilical linecomprises a first fiber optic cable and a second fiber optic cable; anda single photon detector connected to the second fiber optic line. 12.The interrogator of claim 11, further comprising a proximal circulatorlocated between the one or more lasers and the first fiber optic cable.13. The interrogator of claim 11, further comprising a cooling mechanismconnected to the single photon detector.
 14. The interrogator of claim11, further comprising one or more piezoelectric inputs.
 15. Theinterrogator of claim 11, wherein the first fiber optic cable and thesecond fiber optic cable are attached to a distal circulator and thedistal circulator is disposed in the umbilical line.
 16. A method foroptimizing a sampling frequency comprising: identifying a length of afiber optic cable connected to an interrogator, wherein the interrogatorincludes a single photon detector configured to receive a backscatter bydetection with the single photon detector; identifying one or moreregions on the fiber optic cable in which a backscatter is received; andoptimizing the sampling frequency of a distributed acoustic system (DAS)by identifying a minimum time interval that is between an emission of alight pulse such that at no point in time the backscatter arrives backat the interrogator that corresponds to more than one spatial locationalong a sensing portion of the fiber optic cable.
 17. The method ofclaim 16, wherein the fiber optic cable comprises an umbilical lineconnected to a downhole fiber through a fiber connection.
 18. The methodof claim 16, further comprising determining an optical energy of thebackscatter.
 19. The method of claim 16, wherein the fiber optic cablecomprises an umbilical line and the umbilical line comprises a firstfiber optic cable and a second fiber optic cable both attached to adistal circulator.
 20. The method of claim 16, wherein the interrogatorcomprises one or more receivers.